A MIMO system supports multiple-input multiple-output transmission from multiple transmit antennas to multiple receive antennas. Certainly, the MIMO system is also compatible with Single-Input Single-Output (SISO) transmission and the like.
In the MIMO system, generally, a wireless channel between a base station and a terminal needs to be estimated to restore data transmitted over the wireless channel Generally, a pilot signal is transmitted from the base station, and the pilot signal is received by the terminal to perform a channel estimation. The pilot signal is composed of a pilot sequence that is learnt by both the base station and the terminal in advance, and therefore the terminal can perform the channel estimation based on the received pilot signal and the pilot sequence learnt in advance. In the MIMO system, functions of performing the channel estimation by using the pilot signal are categorized into two types:
One type is performing channel sounding, used to enable the terminal to estimate Channel State Information (CSI), including Channel Quality Indicator (CQI) estimation, rank information estimation, and Pre-Coding Indication (PCI) information estimation, and the like. After completing the CSI information estimation, the terminal feeds back the CSI information to the base station over an uplink feedback channel; and the other type is used for data demodulation.
In a 4 Branch MIMO system, that is, a 4×4 (4-input 4-output) MIMO system, a 4×2 (4-input 2-output) MIMO system, or a 4×1 (4-input 1-output) MIMO system, pilot signals are required to be transmitted over four antennas of a base station to support channel estimations performed by a 4 Branch MIMO terminal (including a 4×4 MIMO terminal, a 4×2 MIMO terminal, or a 4×1 MIMO terminal), so that the terminal may further perform a CSI estimation and data demodulation. If power for transmitting a pilot signal transmitted over a third or a fourth antenna is too high, however, a great interference may be caused to a traditional terminal, thereby affecting performance of the traditional terminal.
Therefore, how to reduce interference to the traditional terminal caused by transmitting a pilot signal over the third or the fourth antenna while ensuring performance of the 4 Branch MIMO terminal becomes a major factor that needs to be considered during pilot design for the 4 Branch MIMO system.